The present invention generally pertains to radar systems and is particularly directed to an improvement in bistatic passive radar systems for enabling determination of the distance between two airborne aircraft. Ordinary (monostatic) radar systems have a transmitter and a receiver located at the same site. The transmitter emits electromagnetic radiation signals having a time reference base, such as pulses (radar signals), and the receiver detects reflected radiation from targets illuminated by the transmitted radar signal. The range of the target may be determined by the time it takes a pulse of a electromagnetic radiation to travel from the transmitter to the target and then by reflection from the target back to the receiver. The transmitted pulses are focused in a narrow beam, and the bearing of the target is determined by the bearing of the transmitter's antenna at the time the reflected pulse is received.
When used in military aircraft, monostatic radar has the disadvantage that the transmitter can be detected at long range (hundreds of miles) by the electromagnetic pulses it emits. This allows the enemy to detect the presence of an aircraft and also to determine its bearing. To get around this disadvantage, bistatic passive radar was developed. Bistatic passive radar does not have a transmitter but rather has a receiver system that utilizes the radiation emitted by any monostatic radar system in its reception area. The transmitter of a monostatic radar system which is being used by a bistatic passive radar system is known as the host transmitter. In a bistatic passive radar the system locks onto the host transmitter's pulse train, measures the rotational speed of its antenna and its bearing angle, and typically generates a plan position indicator (PPI) display from this data. Target returns are displayed on the PPI display which has the host transmitter as its center. The present invention does not depend upon or utilize rotational speeds, azimuth bearing angles or PPI display.
With bistatic passive radar systems, the distance between the host transmitter and the bistatic radar system cannot be determined by a simple time measurement as with a monostatic radar system, because real time data indicating when the radar signal was transmitted is not available to the bistatic radar system.
Systems that purportedly determine the distance between two airborne aircraft wherein a first aircraft contains a bistatic passive radar system and the other aircraft contains a host transmitter have been described in U.S. Pat. Nos. 2,837,738 to Van Valkenburgh and 3,939,476 to Leopard et al. In these systems, the distance between the two aircraft is said to be determined by measuring (1) the interval between receipt by the bistatic radar system transmitted radar signals received directly from the host transmitter and receipt by the bistatic radar system of radar signals that are reflected from the ground; (2) the altitude of the first aircraft; and (3) the angle of elevation of the second aircraft from the first aircraft, and by computing the distance from these measurements. However, these systems are not always accurate because the computations are based upon insufficient measured data.